It is generally known that hydraulic media, such as hydraulic oils, change their viscosity according to their temperature. In a hydraulic control mechanism this leads to undesirable control and regulation behavior, particularly when the operating temperature of the control mechanism and, therefore, of the control pressure medium vary within a comparatively wide temperature range. Such an operating behavior is to be detected, for example, in a hydraulic transmission control mechanism in a motor vehicle which, as a rule, is placed close to the transmission housing, the same as near the internal combustion engine. It is thus not surprising that in such a device the temperature of the control fluid may range from −40° C. to 180° C.
Springs can be used to compensate for the described temperature-dependent viscous behavior of the control pressure medium, for example, in the hydraulic control devices of automatic transmissions. The springs, made of a metallic alloy with so-called memory effect, act upon the slide valves located there and assume specific geometries depending on the spring temperature thus compensating, by spring forces of different magnitudes, the described characteristics specific to the temperature of the control pressure medium. Springs of variable spring tension can also be used to compensate for the described characteristics. These springs of variable spring tension are so-called wax tensile material elements. Finally, it is also possible to use pressure nozzles at adequate places in the pressure lines of one such hydraulic control apparatus.
These pressure nozzles have cross-sections that change depending on the temperature (so-called thermal nozzles).
In these known compensation means, their altogether insufficient operating safety regarding the durability of a motor vehicle is disadvantageous. Besides, springs with memory effect disadvantageously have only one defined shift point with clearly distinct hysteresis.
The technical problem as described above, especially with regard to hydraulic control mechanisms, has been satisfactorily solved. The solution is described herebelow with the aid of a concrete example or the explanation of which FIG. 1 has been appended to the description.
In the operation of a motor vehicle having an automatic stepped variable speed transmission of an electronic control and regulation device, the same as of a hydraulic control mechanism controlled by the device, the situation may arise where the electric system of the motor vehicle fails. In such a case, the hydraulic control mechanism of that automatic transmission must be capable, independently of the electric control signals, to prepare and carry out 1) a towing operation of the vehicle; 2) to open the transmission with regard to torque transfer, and 3) accordingly bring it to an idling state.
FIG. 1 illustrates that one such uncompensated shift valve at respective constant temperature of the control pressure medium has a control pressure curve depending on engine rotational speed. At a pressure medium temperature of 0° C. and a rotational speed of about 500 U/min with a control pressure of P_Sek=16 bar, according to the curve a, the control valve is fully open while the curves b, c, d, e, f, for the pressure medium temperatures of 20° C., 40° C., 60° C. and 100° C. make clear that this state occurs at rising temperature and continuously higher rotational speeds N_mot. As shown in a comparison between the two curves a and f, there is a difference in rotational speeds of 800 U/min, upon reaching the maximum control pressure P_Sek, especially at low rotational speeds of the engine. This is an undesirably big difference.
FIG. 1 illustrates that one such uncompensated shift valve at respective constant temperature of the control pressure medium has a control pressure curve depending on engine rotational speed. According to the curve a, at a pressure medium temperature of 0° C. and a rotational speed of about 500 U/min with a control pressure of P_Sek=16 bar, the control valve is already fully open while the curves b, c, d, e, f, for the pressure medium temperatures of 20° C., 40° C., 60° C. and 100° C. make clear that this state occurs at rising temperature and continuously higher rotational speeds N_mot. As shown by a comparison between the two curves a and f, there are between reaching of the maximum control pressure P_Sek about 800 U/min, which is an undesirably big difference specifically at low rotational speeds of the engine.
With this background, the problem to be solved by the invention is, therefore, to introduce an apparatus by means of which a shifting behavior of the shift valve is not temperature dependent or is at least less temperature dependent than has been hitherto usual such that, for example, in the described emergency operation situation, the automatic transmission can be shifted to idling a speed in the proximity zone of a previously established rotational speed of the engine.